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For part of an exam question, "why do cells grown in liquid cause turbidity?" I answered that cells refract light. The correct answer was that cells scatter light. Isn't the light scattering ultimately caused by lots of refraction events? Is the grader correct to take off full points?

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  • $\begingroup$ Cells also reflect and absorb light, but this is really a physics question. $\endgroup$ – canadianer Dec 16 '14 at 23:34
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You're both correct. Turbidity is causes by excessive light reflection events from all of the particles, and is actually how it is measured in a sample. As to cells, they both both reflect light that is incident on a membrane, but they will also refract light that does manage to enter the cell because of the difference in refractive indices. A portion of the light will also be absorbed by the cells.

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  • $\begingroup$ Scattering = refraction + reflection. The third optical event is absorption. Cells also absorb light. Does that contribute to turbidity? $\endgroup$ – clay Dec 17 '14 at 4:27
  • $\begingroup$ Scattering alone contributes to turbidity. Absorbance just causes attenuation of the amount of light (measured in flux) passing through the sample. The light coming out the other side is dimmer than what went in. $\endgroup$ – user560 Dec 18 '14 at 0:48

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